A power takeoff (PTO) is a mechansism commonly used on trucks, tractors, militiary vehicles, and a wide variety of vehicles used for agriculture, mining, heavy construction, and industry for transferring power from an engine to any of a variety of accessories such as hydraulic pumps, pulleys, winches, jackshafts, and the like. In each case, the PTO consists essentially of a small power transmission that is operatively connected to the main transmission of the vehicle. A gear of the PTO, which may be referred to as the PTO drive gear, engages one of the driven gears of the transmission and is usually continuously driven while the engine is operating unless the clutch is disengaged.
Within the housing of the PTO is a pinion gear capable of being shifted into and out of intermeshing engagement with the PTO's drive gear, an operation generally performed by movement of an external lever operatively connected to the pinion by means of a shift fork. In a typical PTO application, such as in a dump-type truck, the dumper box is raised and lowered by a hydraulic cylinder connected to a pump. With the vehicle engine operating and the lever shifted so that the PTO's pinion gear engages the rotating PTO drive gear, the pump delivers hydraulic fluid to or from the cylinder to lift or lower the dumper box..
The prior art includes both direct and remote means for shifting a PTO. In simpler constructions, an extension arm is connected directly to the PTO's operating lever so that the pinion gear may be shifted into and out of engagement with the drive gear by manually moving the extension arm. While such an arrangement has the advantages of simplicity, it also has the disadvantage of requiring a vehicle operator to leave the cab of the vehicle, with the engine running and the controls of the vehicle unintended, during PTO operation. Also, in some installations the location of the PTO lever may make it difficult if not impossible to attach a manually-operable extension arm. Known remote control systems include mechanical linkages, flexible cables, and compressed air systems. While such systems all offer the advantages of remote operation, those advantages may be offset to a greater or lesser extent by a variety of disadvantages. Thus, mechanical linkages are usually expensive to make and install because they are often custom fabricated for each type of vehicle and may be difficult and time consuming to attach and adjust. Also, such linkage systems tend to be susceptible to damage, with even minor deformations or maladjustments rendering such systems inoperable.
Push-pull controls of the flexible cable type are usually less expensive than rigid linkage systems but they are also costly to install since such a control cable and its sheath must be carefully routed through the vehicle, and final adjustments must be carefully made, to achieve proper operation. Should the cable become kinked in use, or should the manual control be abused by rough handling, inoperability of such a system may easily result.
Compressed air systems require relatively expensive cylinders, valves, and high pressure hoses. While such a system is relatively simple, it demands a vehicle equipped with an air compressor and it also necessitates careful routing of all hoses so that they will not be exposed to heat or abrasion.
Other systems that have been attempted include hydraulic systems and electromechanical shifters. Hydraulic systems have been largedly rejected because of their high expense, and electromechanical shifters, including solenoids, electric motors, and linear actuators of one type or another have been found generally unacceptable because of unreliability and insufficient operating life.
One aspect of this invention lies in the discovery that previous efforts to utilize electromechanical means in remote control systems for PTOs have generally failed because they did not take into full account the high loads and gear-damaging impact forces that develop should the teeth of a PTO pinion gear fail to mesh instantly with those of a PTO drive gear. Ordinarily, such gears are mounted on parallel shafts with the pinion gear being shifted laterally or axially into and out of engagement with the rotating drive gear. Often the gears do not mesh smoothly but rather toothface hits toothface for a jam condition referred to as a "clash" shift. Mechanisms capable of shifting the pinion with considerable force, including hydraulic and electromechanical systems, would be expected only to exacerbate the problem by increasing the likelihood of wear and damage to the teeth of the gears under repeated clash conditions.
This invention also lies in the further discovery that an electromechanical linear actuator, despite its relatively slow operation and the high forces it is capable of generating, may be ideally suited for use in shifting the pinion of a PTO into meshing engagement with a rotating drive gear if some means could be provided for storing energy from the moment of clash contact and until tooth alignment occurs, at which time the stored energy might be instantly released to shift the gears together. Under such circumstances, the potentially damaging effects of a clash shift would be avoided.
The electromechanical shifter of this invention is used in combination with a PTO having a housing adapted for connection to a vehicle transmission and containing a drive gear that is rotated by the transmission when the transmission is in operation. The PTO also includes a pinion gear mounted for axial movement in the housing between engaging and disengaging positions relative to the drive gear. Briefly, the electromechanical shifter includes a shock shaft having telescoping first and second sections each mounted for axial movement relative to the housing. A positioning arm, which may include a fork portion at its end, is carried by the second section and is engagable with the pinion for urging it towards intermeshing engagement with the drive gear when the second section is shifted axially in one direction. Electrically-powered linear actuator means is coupled to the first section for progressively shifting the same axially in said one direction when the actuator means is energized. Compression spring means is operatively interposed between the first and second sections of the shock shaft for gradually storing increasing levels of energy by compressing when the pinion gear is shifted axially into clashing contact with the drive gear and for quickly releasing such stored energy to move the pinion gear into intermeshing engagement with the drive gear when a condition of alignment for allowing such intermeshing engagement occurs.
Means are also provided for limiting the maximum force that the linear actuator may apply in a clash situation and the maximum duration of the interval of force application. In the embodiment disclosed, both functions are performed by the electrical circuitry of the linear actuator, with a transistor controlling the maximum electrical load under clash conditions and an RC time delay circuit limiting the duration of operation of the actuator, to insure that even under conditions of less than maximum load a clash condition cannot exist for more than a predetermined interval.